CA2764270A1

CA2764270A1 - Fuel channel spacer system and method

Info

Publication number: CA2764270A1
Application number: CA2764270A
Authority: CA
Inventors: James M. King; Paul G. Adams
Original assignee: Atomic Energy of Canada Ltd AECL
Current assignee: Atomic Energy of Canada Ltd AECL
Priority date: 2012-01-13
Filing date: 2012-01-13
Publication date: 2013-07-13
Anticipated expiration: 2032-01-13
Also published as: WO2013104052A1; CA2764270C

Abstract

The present invention relates to a spacer for maintaining a distance between a pressure tube and a calandria tube in a nuclear reactor, and more specifically, to a spacer which is secured in position between the pressure tube and calandria tube. The axial position of the spacer is maintained by having a spacer with an outer profile that is in close fit or a slight interference fit with a locally expanded profile of the calandria tube. A spacer, a corresponding calandria tube, and a method for installing such a spacer are described.

Description

FUEL CHANNEL SPACER SYSTEM AND METHOD
FIELD OF INVENTION
[0001] The present invention relates to a spacer for maintaining an inner tube in spaced relation within an outer tube and in particular to a spacer for maintaining a distance between a pressure tube and a calandria tube in a nuclear reactor. The invention is particularly concerned with a spacer which is secured in position between the pressure tube and calandria tube, and a spacer with simple installation and replacement.
BACKGROUND OF THE INVENTION

[0002] Referring to Figures 1 and 2, one of the major components of a nuclear reactor is a calandria - a large, sealed tank, in which the nuclear reaction takes place.
The calandria is penetrated by many tubes (i.e. calandria tubes) allowing uranium or similar fuel bundles to be inserted into the calandria via fuel channels, and allowing pressure tubes to draw heat to feed the generation system. In a CANDU reactor, a fuel channel consists of a 104 mm diameter, 4.3 mm thick zirconium alloy pressure tube, inserted into a calandria tube of a slightly larger diameter, with two stainless steel end fittings at the ends of the fuel channel.
Several hundred calandria tubes, approximately 6.3 m long, are horizontally mounted in the calandria.

[0003] The annular gap between the pressure tubes and calandria tubes are filled with CO2 gas which acts as an insulator between the "hot" pressure tube and the heavy water moderator in the calandria vessel. Heavy water flows through the pressure tubes, removing heat from the fuel bundles and transferring it to steam generators, where secondary circuit light water is heated and converted into steam to run a turbine. During reactor operation, pressure tube material is subject to high pressure (up to 11.3 MPa), high temperature (up to 310 C) and very high gamma and neutron radiation fields. The calandria tubes are subjected to head pressure (from the moderator located in the calandria).

[0004] As the inner pressure tube operates at a relatively high temperature and the outer calandria tube operates at a much lower temperature, fuel channels in a nuclear reactor, - -such as a CANDU reactor, require an annular space to be maintained between the pressure tube and the coaxial calandria tube in order to maintain the temperature differential, to allow for the circulation of gases which thermally insulate the hot pressure tube from the relatively colder calandria tube and the heavy water moderator which flows in the space outside the calandria tube.

[0005] However, the weight of the fuel and coolant inside the pressure tube would cause it to sag into contact with the calandria tube if it were not supported at a few discrete points along its span. Annulus spacers are designed for transmitting supporting force and maintaining the required gap between the pairs of calandria and pressure tubes.

[0006] Therefore, annulus spacers are an important component that makes up a reactor fuel channel. These spacers maintain the radial spacing between the two coaxial tubes (the inner pressure tube and the outer calandria tube) and help the calandria tubes to support pressure tubes. Typically, four spacers are used in each fuel channel, each at a different axial position. To provide the required support of the pressure tube, the annulus spacers must be located at the proper position. If a spacer is out of position, the hot pressure tube may come into contact with the cooler calandria tube, which is unacceptable in the reactor.

[0007] Conventionally, garter spring spacers have been used to maintain the space between the pressure tube and the calandria tube. A garter spring spacer is basically a helical spring disposed around the pressure tube. Its convolutions contact the walls of both the pressure tube and the calandria tube. The spring is unattached to either tube. A garter spring spacer was disclosed in United States Patent No. 3,106,520 issued to Wolfe et al.
October 8, 1963.

[0008] Two types of such garter spring spacers have been used in CANDU fuel channels, known as the loose-fit spacer and the snug-fit spacer. Their arrangements are shown in Figure 3. Both garter spring spacers comprise a closely coiled spring made from a square cross section wire, assembled on a circular girdle wire to form a torus. The design of the annulus spacer is such that they are not fixed rigidly in position. It is possible that a spacer may move out of position.

[0009] The loose-fit garter spring spacer does not reliably remain in its installed location.
The loose-fit garter spring spacer relies on friction to maintain position.
Some loose-fit garter spring spacers move axially away from their intended positions during reactor operation thereby causing a major source of concern, as the spacers must stay in their intended positions to ensure that the pressure tube is adequately supported and remains out of contact with the calandria tube. The loose-fit garter spring design has been replaced with the snug-fit garter spring design for better performance in maintaining its axial position along the fuel channel.

[0010] The snug-fit garter spring has been shown to be more reliable in maintaining its installed location in the reactor. This is initially done by spring tension on the pressure tube. Over time the spring tension decreases and the garter spring becomes pinched between the pressure tube and calandria tube, which aides in keeping the snug-fit garter spacer in position through friction. The snug-fit garter spring is typically made using Inconel X-750 for the helical spring coil and Zircaloy-2 for the girdle wire.
Inconel X-750 is a nickel-based alloy, and it was chosen in part for its ability to maintain the required spring tension under the given operating conditions. However, it has been discovered that the mechanical properties of nickel-based alloys degrade with prolonged exposure to radiation, causing concerns about the condition of the Inconel garter springs over time.

[0011] An additional drawback is that the snug-fit spacer is difficult to detect to confirm its position. Loose-fit garter spring spacers can be detected relatively easily using eddy current technology by detecting an induced current in the welded girdle wire.
The snug-fit garter spring cannot be detected using the eddy current technique because it does not have a continuous uninterrupted circuit around its perimeter, as its girdle wire is not welded.
Techniques based on inspecting for pressure tube deformation (sag, ovality, pressure tube-to-calandria tube gap) have been used to indirectly identify spacer position.
Recently, a vibration-based technique (termed MODAR for MOdal Detection And Repositioning) has been developed to detect snug-fit garter spring position by monitoring the effect the spacer load has on controlled pressure tube vibrations.

[0012] Thus, neither the loose-fit nor snug-fit garter spring spacer are positively located in the fuel channel. In fact, the garter spring spacer is designed to roll when relative axial motion occurs between the pressure tube and calandria tube due to thermal changes and creep, so its position changes under operating conditions. This characteristic that the spacer position is not fixed results in the regulator requiring that reactor operators perform inspections to verify spacer position. These inspections add cost and decrease operating efficiency of the reactor.

[0013] When a change to the spacer axial location occurs, the spacer must be repositioned. Repositioning the spacers is difficult and costly, and may also result in radiation exposure to those who conduct the procedure.

[0014] Further, because garter spring spacers are not attached to either the pressure tube or the calandria tube, they must be installed on the pressure tube after the pressure tube has been placed inside the calandria tube. As a result, installation of the garter spring spacers is a challenging procedure which requires tedious operations to be carried out at the reactor face. The problem is exacerbated over the operating time of the fuel channel as increased sag develops in the calandria tubes. Spacer installation in a sagged fuel channel is significantly more challenging than in a straight fuel channel.

[0015] The difficulty in installing the spacers is of particular significance to the fuel channel replacement procedures because when a fuel channel is replaced, the spacers must be re-installed. Consequently, this adds to the time and cost of fuel channel replacement.
An improved fuel channel spacer replacement procedure is desirable not only to reduce the time and expense of the operation but also to reduce the radiation dose level to which those who replace the fuel channels may be exposed.

[0016] It would be desirable to use only low neutron cross section material, such as zirconium alloy, in a spacer design, instead of Inconel, to reduce fuel burn-up and increase neutron efficiency.

[0017] There is therefore a need for an improved spacer which is positioned between the pressure tube and calandria tube. It is also desirable that the improved spacer overcomes some of the difficulties inherent in the use of prior art spacers such as the garter spring spacer.
SUMMARY OF THE INVENTION

[0018] It is an object of the invention to provide an improved spacer design and in particular, to provide a spacer design that is fixed in space and cannot be easily moved out of position, and that can be easily installed and/or replaced.

[0019] Presently, none of the current fuel channel annulus spacers are 'positively' located in the fuel channel (i.e. there is nothing that physically keeps the spacers in position or prevents them from moving out of position). When spacers move away from their intended positions during reactor operation, a major concern arises as to whether the pressure tube is adequately supported and remains out of contact with the calandria tube.

[0020] According to an aspect of the present invention there is provided a spacer for maintaining a pressure tube in spaced relation with a calandria tube in a nuclear reactor, wherein an outer profile of the spacer has a close fit with a locally, circumferentially-expanded profile of the calandria tube. This prevents axial movement of the spacer within the calandria tube.

[0021] According to another aspect of the present invention there is provided a calandria tube in a nuclear reactor comprising a locally, circumferentially-expanded profile for securing a spacer.

[0022] According to a further aspect of the present invention there is provided a method of installing a spacer in a nuclear reactor, comprising the steps of: positioning the spacer at an end of a calandria tube, the spacer being rotated 90 degrees out of installed position about its vertical axis; inserting the spacer into the calandria tube to align axially with a formed profile of the calandria tube; rotating the spacer 90 degrees about its vertical axis to engage with the formed profile of the calandria tube, and inserting the pressure tube through the calandria tube and spacer, fixing the axial position of the spacer.

[0023] Other systems, methods, features and advantages of the invention will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the following claims.
BRIEF DESCRIPTION OF THE DRAWINGS

[0024] These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings wherein:
Figure 1 presents a schematic diagram of a nuclear reactor as known in the art;
Figure 2 presents a simplified diagram of a calandria and immediately related components as known in the art;
Figure 3 presents an arrangement of calandria tube, pressure tube and garter spring annulus spacers as known in the art;
Figures 4A and 4B present orthogonal and isometric cutaway views, respectively, of the spacer in accordance with an embodiment of the present invention;
Figures 5A and 5B present the details and dimensions of the spacer body in accordance with an embodiment of the present invention;
Figures 6A through 6D present schematic views of the spacer installation process in an embodiment of the present invention;
Figures 7A through 7D present isometric views of the spacer installation process in an embodiment of the present invention;

DETAILED DESCRIPTION

[0025] One or more currently preferred embodiments have been described by way of example. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.

[0026] As noted above, a key problem with the current fuel channel annulus spacer is that it is not 'positively' located in the fuel channel, that is, there is nothing that physically keeps the spacer in position or prevents it from moving out of position.
Further, installation of the current spring spacers is a challenging procedure which requires additional operations to be carried out at the reactor face. Due to the above-described issues, there has been a long felt need in the industry for a new fuel channel spacer to replace the garter spring type spacer. Several concepts have been proposed but heretofore, none have been successful. The new fuel channel annulus spacer design provided by the invention has been developed to address the performance shortcomings of the current garter spring type spacer.

[0027] The current invention provides a novel spacer design that is fixed in space and cannot be easily moved out of position. It can also be easily installed and/or replaced. The spacer design also involves a change to the profile of the calandria tube such that the revised profile is used to help fix the spacer position. Universal adoption will replace the existing garter spring type fuel channel spacer.

[0028] In the preferred embodiment of the invention, a novel and elegant solution to fixing the position of the annulus spacer in the fuel channel is provided by capturing the spacer between the pressure tube and a locally expanded section of calandria tube.
With the system of the invention, fixing the spacer requires no attachments (welds, threaded features, etc.) to either the pressure tube or calandria tube. The locally expanded sections of the calandria tube may be formed using a known industrial process called hydroforming.
This process uses hydraulic pressure to apply force to shape the calandria tube. In this case, a shaped die with the desired form is fit around the outside of the straight calandria tube at the desired location and orientation. The die is split to allow it to be removed after hydroforming, and has a collar installed around it to hold it securely together during the expansion process. A type of plug is installed inside the calandria tube, coincident with the location of the die on the outside. At each end of the plug a seal is formed against the inside surface of the calandria tube. The plug includes appropriate ports to allow hydraulic fluid to be introduced into the sealed annulus between the plug and calandria tube. The fluid is then pressurized, which exerts enough force to cause the calandria tube to expand outward against the formed die. The calandria tube is left permanently deformed by this process. The jigs required would be specially made to fit the calandria tube and to produce the desired shape. Note that it would not be feasible to hydroform old calandria tubes: new tubes are invariably used when building a new reactor or refurbishing an old reactor.

[0029] Note that with the expansion of the calandria tubes to accommodate the spacer, the holes through the calandria vessel generally do not have to be made larger.
This is because the profile of the expanded section of calandria tube is designed such that it is within the envelope of the larger diameter belled ends that typically exist on the calandria tubes.
Thus, no modification to the calandria vessel or the connection between the calandria and calandria tubes is generally required by the new spacer design.

[0030] The preferred embodiment of the spacer design is shown in Figure 4. The spacer consists of a body in the form of a generally circular- or ring-shaped band, which is flattened on the two vertical sides (i.e. left side and right side). The lower portion of the spacer body provides support for a set of rollers. The rollers are secured to the body using pins. In use, the spacer is positioned inside the calandria tube and the pressure tube rests on the rollers. A roller-type support helps to minimize friction and possible wear when relative axial motion occurs between the pressure tube and calandria tube. It also helps prevent displacement of the spacer when a pressure tube is being inserted into a calandria tube.

[0031] The outer profile of the spacer body is specifically designed to facilitate it being securely positioned inside the calandria tube at locations where a special profile has been formed. The profile of the spacer body is shown in the section view of Figure 5A. The outside dimension of the spacer at the vertical and horizontal centerlines of its body approximately matches that of the inside diameter of the calandria tube. On the flattened vertical sides of the spacer, the width is maintained for a small distance above and below the horizontal centerline, producing the two flat portions shown in the section view. The curved portions between the vertical centerline and the flattened portions are sized to have a radius larger than the inside diameter of the calandria, and slightly smaller than the inside diameter of the locally expanded portion of the calandria tube. Thus, when the spacer is in its final position within the expanded section, it will not be possible to displace the spacer axially because of interference with the unexpanded calandria tube. The width of the flattened portions is determined simply by the amount desired to extend the outside spacer profile beyond the calandria tube body inside diameter so as to secure it in place.

[0032] The actual three dimensional profile along the vertical side portion of the spacer is cylindrical, with the cylindrical axis coinciding with the spacer centre. This rounded cross-section of the flattened portions is shown in the cross-sectional view of Figure 5B.
Having the side flattened portions rounded, allows the spacer to be rotated about its vertical axis once the spacer is in the locally expanded portion of the calandria tube, the curvature of the side flattened portions matching that of the expanded portion of the calandria tube. This profile also prevents the spacer from rotating about either the axis of the calandria tube or about a horizontal axis perpendicular to the calandria tube axis.

[0033] Figures 6 and 7 illustrate the installation process, Figures 6A through presenting a schematic cross-sectional view from above, and Figures 7A through presenting isometric views.

[0034] To install the spacer, it is initially held vertically but rotated 90 degrees about the vertical axis from its installed position, at the end of the calandria tube, as shown in Figures 6A and 7A. The spacer is then inserted into the calandria tube until it is aligned axially with the formed profile of the calandria tube (i.e. the expanded portion of the calandria tube) as shown in Figures 6B and 7B.

[0035] The spacer is then rotated 90 degrees about the vertical axis to engage it with the formed calandria tube matching profile as shown in Figures 6C and 7C. The flattened sides of the spacer are rounded so that their curvature matches the curvature of the expansion in the calandria tube, facilitating the easy rotation of the spacer into its final position. The pressure tube can then be installed in the calandria tube and through the spacer. The presence of the installed pressure tube prevents the spacer from rotating about its vertical axis. With the pressure tube installed, the spacer is fully captured in place.

[0036] A significant feature of the invention is that the calandria tube is locally formed (expanded) to allow the spacer to be fixed in position. The formed calandria tube makes many alternate means of designing and fixing a spacer approach possible. For example, the spacer could be hinged to expand into the formed shape and then be pinned in the opened configuration to fix it in place. The preferred embodiment detailed above is provided for its simplicity of installation. However, other design variations based on the same approach of expanded forming of the calandria tube are also included in the scope of the invention.

[0037] The new spacer design has many advantages over the conventional designs. For example, it secures the spacer in position by capturing it between the pressure tube and calandria tube, therefore it physically keeps the spacer in position and prevents it from moving out of position. Further, the spacer can be easily installed into position inside of the calandria tube. There are no special attachment features that would need to be inspected or remotely manipulated. The spacer can easily be removed and re-installed if needed for any practical reason.

[0038] A further advantage of the design of the spacer of the invention is that it is mechanically robust and does not require materials of construction to be chosen based on resistance to tension or any other requirement originating from the spacer design itself.
The spacer may be made of zirconium alloys. This will reduce fuel burn-up and increase neutron efficiency for the reactor. Suitable oxide coating on the pins and rollers may be used to improve the wear characteristics of the design and reduce friction.

[0039] There is a huge economic advantage of producing an improved fuel channel annulus spacer as described in the present invention. The current spacers do not have a fixed position. This has caused concerns from nuclear regulators and has resulted in very large expenditures related to inspections and assessment of the effects of variations in spacer positions. The issues with the existing spacer design are widely known and are important considerations for potential buyers of nuclear reactors.

[0040] Implementation of the fuel channel annulus spacer design of the invention will improve the operating performance of reactors such as CANDU reactors and may reduce the need for costly inspections. Of course, the new design may be applied to new build nuclear reactors and for refurbishment projects (i.e. reactor re-tubing).
OPTIONS AND ALTERNATIVES

[0041] Many variations to the described spacer are possible. Examples of variations include an alternate number of rollers (2 or 5 instead of 4), adding rollers to the top portion of the spacer in addition to the bottom, use of a solid bearing instead of a roller, altering the spacer installation axis away from vertical, changing the materials of construction, or using a hinge or latch feature to extend part of the spacer into the expanded calandria tube as a means to secure it in place.
CONCLUSIONS

[0042] One or more currently preferred embodiments have been described by way of example. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.

[0043] All citations are hereby incorporated by reference.

Claims

1. A spacer for maintaining a pressure tube in spaced relation with a calandria tube in a nuclear reactor, wherein an outer profile of the spacer is generally circular-shaped, and has a close fit or slight interference fit with a locally expanded profile of the calandria tube.

2. The spacer of claim 1, wherein the outer profile of the spacer is frictionally engaged with the locally expanded profile of the calandria tube, and the outer profile of the spacer is greater in diameter than the inside diameter of the calandria tube.

3. The spacer of claim 1, wherein the body of the spacer comprises a generally circularly-shaped body.

4. The spacer of claim 3, wherein a width of the spacer at one of vertical and horizontal centerlines of the generally ring-shaped body approximately matches an inside diameter of the calandria tube.

5. The spacer of claim 4, wherein the width of the spacer at the one centerline is maintained for small distance extending from the horizontal centerline, producing a vertical flat portion in section view.

6. The spacer of claim 5, wherein the outer profile of the spacer at the vertical flat portion includes a cylindrical surface.

7. The spacer of claim 6, wherein axis of the cylindrical surface coincides with the vertical center of the spacer.

8. The spacer of claim 1, further comprising rollers that are secured to the body using pins.

9. The spacer of claim 8, wherein the rollers are located at the lower portion of the spacer.

10. A calandria tube in a nuclear reactor comprising a locally expanded profile for securing a spacer.

11. The calandria tube of claim 10, wherein the locally expanded profile is cylindrical along the straight vertical portions of the corresponding spacer profile.

12. The calandria tube of claim 10, wherein the locally expanded profile comprises four variable-radius expanded portions, nominally centred at 45 degrees from the vertical and horizontal axes, whose outer surface is radiused about the spacer vertical centre line.

13. A spacer in a nuclear reactor comprising an outer profile of the spacer matching the locally expanded profile of the calandria tube according to claim 6.

14. A method of installing a spacer in a nuclear reactor, comprising the steps of:
positioning the spacer at an end of a calandria tube, the spacer being rotated degrees out of installed position about its vertical axis;
inserting the spacer into the calandria tube to align axially with a formed profile of the calandria tube;
rotating the spacer 90 degrees about its vertical axis to engage with the formed profile of the calandria tube; and inserting the pressure tube through the calandria tube and spacer, fixing the axial position of the spacer.

15. The calandria tube of claim 10 wherein the profile is only cylindrical in the area that corresponds to the vertical flat sections of the spacer, centred at the 3 and 9 o'clock positions, the locally expanded profile having a varied radius, centred on the spacer vertical centerline, that matches the curved profile between the side vertical flat sections and the top and bottom locations coincident with the vertical centreline.